Se encuentra usted aquí
Organic matter in sediments of Antartic Continental Shelves under the influence of Climate Change
In the last 60 years air and oceanic temperatures increased in Antarctica above global average (Vaughan et al., 2001; Gille, 2002). As a consequence, the increase of glaciers retreat (Cook et al., 2005) and the thinning of ice shelves (Shepherd et al., 2003; Whingham et al., 2009) have been observed. Furthermore, ice shelf collapses became more frequent in Antarctica, as exemplified by the collapse in 2008 of Wilkins ice shelf and in 2010 of Mertz glacier. In 1995 and 2002, 4200 km2 and 3200 km2 of the sections A and B of the Larsen Ice shelf disintegrated, respectively (Rack and Rott, 2004). Larsen A ice shelf disintegration was preceded by three retreat episodes in 1400, 2100 and 3800 year BP (Brachfield et al., 2003), whereas ice shelf Larsen B started collapsing in 1947 after 10000 years of stability (Domack et al., 2005a). During Antarctic expedition ANT-XXIII/8 we had the opportunity to reach the Bays Larsen A and B off the Eastern Antarctic Peninsula (EAP) and sample the sediment column (the upper 30 cm). One of the objectives of the present work was to study organic matter (OM) accumulation in the sediment before and after ice shelf collapses. It is not usual to have the opportunity to analyze sediment from areas in which primary production and OM flux to the seafloor started just some years before sampling takes place. The bays Larsen A and B represented a natural laboratory where it was possible to investigate the recent arrival of OM onto a rather poor seabed. The quantity and the chemical quality of the OM found in EAP sediments were compared to the characteristics of the OM analyzed in sediments off the Northern Antarctic Peninsula (NAP) and the South Eastern Weddell Sea (SEWS), where no ice shelves developed at least for a thousand years (Ingólfsson et al., 1998; Anderson et al., 2002). Another objective was to identify which OM constituent among pigments (PG), amino acids (AA), fatty acids (FA), total carbohydrates (CHO), total lipids (LPD) and total proteins (PRT), was better to use as biomarker to record environmental changes. A third objective was to identify relationships between sediment chemical characteristics and benthic communities' structure aiming to produce a comprehensive picture of the changing Antarctic ecosystem under extinct ice shelves. To accomplish with this objective abundance, biomass and diversity of the benthic macrofauna from EAP and SEWS were studied. Microscopical counts of diatom valves were carried out on EAP and NAP sediment samples to complete the information on OM sources available from data on amino acids and fatty acids. Excess 210 Pb activity was also measured to develop a radiochemical chronology of sedimentary events in NAP and EAP. In NAP, pigment and amino acid concentrations were higher than in EAP throughout the sediment cores. Diatom valves, 210 Pb activity, pigment and amino acid vertical distributions indicated a lower input of labile OM to the seafloor and a shorter period of accumulation in EAP than in NAP. Labile OM was only found in the upper 2 cm of the sediment column in EAP. The quantity and chemical quality of the biogenic variables throughout the sediment column demonstrated that the presence of ice shelves effectively hamper the vertical input of labile OM to the seabed. Diatom valves and pigment profiles strongly suggested that this is mainly the consequence of rather limited primary production. Recently deposited OM constituents were mainly originated from diatoms in both EAP and NAP. Labile OM was vertically transported from the euphotic zone, while refractory CHO and mid-chained FA most likely accumulated by lateral advection during years of ice shelf coverage and lack of fresh OM supply. In the study area the absence of Chlb confirmed the lack of terrestrial inputs of OM from the Antarctic continent and the absence of iso- and anteiso-FA, hydroxylated FA and 18:1(n-7) evidenced the lack of OM of bacterial origin. Pigments revealed as the best markers to identify the deposition of OM after Larsen A and B ice shelf collapses in EAP. The very low proportion of polyunsaturated fatty acids (PUFA) and the absence of diatom PUFA, in both EAP and NAP reflected a rapid PUFA degradation even in this polar environment and suggested that, in the case of the present study, PUFA cannot be used as indicators of phytoplankton debris or fresh OM in a mid-term (years). Total CHO, PRT, LPD and biogenic silica were not good indicators to track recent changes in the water column because they are not specific to any source of material. Benthic macrofauna in EAP presented relatively low abundance and biomass. Typical pioneer species belonging to the class Polychaeta and the phylum Echinodermata increased diversity in this region. PG, diatom valves and 210 Pb activity profiles demonstrated that this type of benthic assemblage did not produce important bioturbation in subsurface sediment.